1. Field of the Invention
The present invention relates to velocity measurement systems, and more particularly, to acoustic Doppler current profilers, other underwater instrumentation such as Doppler logs, and radar applications.
2. Description of the Related Technology
A current profiler is a type of sonar system that is used to remotely measure water velocity over varying ranges. Current profilers are used in freshwater environments such as rivers, lakes and estuaries, as well as in saltwater environments such as the ocean, for studying the effects of current velocities. The measurement of accurate current velocities is important in such diverse fields as weather prediction, biological studies of nutrients, environmental studies of sewage dispersion, and commercial exploration for natural resources, including oil.
Typically, current profilers are used to measure current velocities in a vertical column of water for each depth “cell” of water up to a maximum range, thus producing a “profile” of water velocities. The general profiler system includes a transducer to generate pulses of sound (which when downconverted to human hearing frequencies sound like “pings”) that backscatter as echoes from plankton, small particles, and small-scale inhomogeneities in the water. Similarly, bottom tracking Doppler velocity logs receive backscattered echoes from the bottom surface. The received sound has a Doppler frequency shift proportionate to the relative velocity between the scatters and the transducer.
The physics for determining a single velocity vector component Vx from such a Doppler frequency shift may be concisely stated by the following equation:
                    Vx        =                              cf            D                                2            ⁢                          f              T                        ⁢            cos            ⁢                                                  ⁢            θ                                              Equation        ⁢                                  ⁢        1            
In Equation 1, c is the velocity of sound in water, about 1500 meters/second. Thus, by knowing the transmitted sound frequency, fT, and declination angle of the transmitter transducer, .theta., and measuring the received frequency from a single, narrowband pulse, the Doppler frequency shift, fD, determines one velocity vector component. Relative velocity of the measured horizontal “slice” or depth cell, is determined by subtracting out a measurement of vessel earth reference velocity, Ve. Earth reference velocity can be measured by pinging the ocean bottom whenever it comes within sonar range or by a navigation system such as LORAN or GPS. FIGS. 1a and 1b show example current profiles where North and East current velocities (Vx, Vy) are shown as a function of depth cells.
In some configurations, current profilers are configured as an assembly of four diverging transducers, spaced at 90° azimuth intervals from one another around the electronics housing. This transducer arrangement is known in the technology as the Janus configuration. A three beam system permits measurements of three velocity components, Vx,Vy, and Vz (identified respectively as u, v, w in oceanographic literature) under the assumption that currents are uniform in the plane perpendicular to the transducers mutual axis. However, four beams are often used for redundancy and reliability. The current profiler system may be attached to the hull of a vessel, remain on stationary buoys, or be moored to the ocean floor as is a current profiler 10 shown in FIG. 2.
Current profilers are subject to trade-offs among a variety of factors, including maximum profiling range and temporal, spatial (the size of the depth cell), and velocity resolution. Temporal resolution refers to the time required to achieve a velocity estimate with the required degree of accuracy. In typical applications, a current profiler will make a series of measurements which are then averaged together to produce a single velocity estimate with an acceptable level of velocity variance, or squared error. In some applications, bias is more of a concern than the variance in observations. Bias is the difference between measured velocity and actual velocity. It is caused, for example, by asymmetries in bandlimited system components. Measurement bias remains even after long-term averaging has reduced variance to a predetermined acceptable limit. For instance, bias dominance is typically found in measuring large-scale features such as those found at temperature and salinity interfaces.
There are many other velocity measurement systems in addition to the current profilers. Some examples are radar systems, air current measurement systems, and other underwater instrumentation such as Doppler logs which measures the velocity of a vehicle or vessel relative to the surface or bottom of a water body. All these velocity measurement systems have a wide range of applications, and it would be beneficial in the art to utilize and/or modify the characteristics of these types of devices so that their features can be exploited in improving existing products and creating new products that have not yet been developed.